Some embodiments of the present disclosure relate generally to bang-bang phase detectors in analog clock data recovery (CDR).
Serial links include a transmitter connected to a receiver via a channel. The receiver generally includes a circuit configured to generate a clock that aligns with the phase of the incoming data.
FIG. 1 depicts a related art CDR circuit configured to align a locally generated clock with an incoming data signal.
Referring to FIG. 1, a related art CDR system 100 includes data and crossing slicers 110 that sample the incoming data and provide the data to the bang-bang phase detector 120. The bang-bang phase detector 120 determines if the phase of the clock generated by a voltage controlled oscillator (VCO) 130 is in alignment (e.g., in phase) with the incoming data. A bang-bang phase detector has 3 states that include the phase being early, late, or that there is no useful information (e.g., there is no data transition required to determine phase alignment). Based on the current state, the bang-bang phase detector 120 outputs an up value or a down value. The up and down signals are provided to a charge pump 140 which activates a switch to increase or decrease the control voltage of the VCO 130. For example, when an up signal is received at the charge pump 140, the charge pump increases the control voltage of the VCO 130 and the speed of the clock generated is increased. Similarly, when a down signal is received at the charge pump 140, the charge pump 140 decreases the control voltage of the VCO 130 and the speed of the clock generated is decreased.
CDRs that use bang-bang phase detectors with charge pumps suffer from a number of problems. For example, the loop needs to have a low bandwidth to filter jitter due to inter-symbol interference (ISI), which requires either a large charge pump capacitor (C) or a small charge pump current (Icp), which causes the charge pump to be sensitive to mismatch, charge injection, and other non-idealities. Additionally, it may be difficult to operate a charge pump at a high frequency, especially with a small charge pump current (Icp) (e.g., the charge injection may overwhelm the Icp).
Thus, a method of decimating the number of ups and downs from the bang-bang phase detector is needed. Related art methods of decimating the output of a bang-bang phase detector often discard information. For example, some systems decimate the bang-bang phase detector by a factor N (e.g., every Nth output is provided). Some systems decimate by adding all the ups and subtracting all the downs for a period of time and outputting the sign of the result. This summing method, however, discards how many more ups than downs or downs than ups occurred. Thus, an improved system and method that requires less power, space, and maintains the fidelity of the number of ups and downs output by the bang-bang phase detector is desired.
The above information is only for enhancement of understanding of the background of embodiments of the present disclosure, and therefore may contain information that does not form the prior art.